In time-division multiple access (TDMA) communication systems, subscriber units communicate with a main station or base site on a basis of non-overlapping transmission bursts that occur in designated time slots. A burst normally consists of a preamble and the information to be transmitted. The preamble usually contains synchronization, control, and routing information. Typically, `n` time slots occur over a time interval called a frame. The duration of the frame and each time slot generally depends on the system. Since transmission bursts do not overlap, a same carrier frequency may be assigned to all subscriber units that are communicating with a same base site. The shared transmission path from subscriber unit to base site is commonly called the uplink channel. The path of transmission from base site to subscriber unit is called the downlink channel. In general, any available time slot can be used by any subscriber unit. In such a system, there is a need for a method of fairly allocating the time slots to users as the number and location of available time slots changes. The process by which multiple users request and are granted access to the shared transmission channel is called contention.
One well known method for accessing a shared communication medium is the slotted ALOHA technique, described, for example, in the book entitled "Computer Networks" by Andrew S. Tanenbaum, Prentice/Hall, Chapter 6, page 253. Because there is no allocation of time slots for specific subscriber units, it is conceivable that concurrent uplink transmission of data packets will occur. The data packet received by the base site will then be a combination of multiple transmissions and will not correspond to any of the individual packets. This action is referred to as a "collision" and requires that each individual subscriber unit attempt to retransmit its data packet. Other variations of the slotted-ALOHA scheme attempt to minimize or eliminate the collision problem by designating or reserving a given time slot or series of time slots for use by a specific subscriber unit (Tannenbaum, page 271).
These schemes have properties that make them inappropriate for some applications and systems. The slotted-ALOHA technique is not efficient for sending large packets that require multiple time slots to transmit. Because each part or segment of the packet is sent independently of the others, significant bandwidth is wasted repeating overhead information, such as identification information, routing information, and information related to packet re-assembly. The unit must also contend before sending each of the segments, meaning more delay. Techniques based on reserving time slots generally require more overhead, particularly on the downlink channel where information is broadcasted that describes who has been granted a reservation and for how long.
Thus, as packet-switched data communication becomes more and more popular, there is a need for developing a method and system for providing more efficient data throughput and minimizing delay in uplink packet transmission.